Hardening of copper to improve copper CMP performance

ABSTRACT

A method for reducing the topography from CMP of metal layers during the semiconductor manufacturing process is described. Small amounts of solute are introduced into the conductive metal layer before polishing, resulting in a material with electrical conductivity and electromigration properties that are very similar or superior to that of the pure metal, while having hardness that is more closely matched to that of the surrounding oxide dielectric layers. This may allow for better control of the CMP process, with less dishing and oxide erosion a result. A secondary benefit of this invention may be the elimination of superficial damage and embedded particles in the conductive layers caused by the abrasive particles in the slurries.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to semiconductor processing technology generally,and more specifically, to chemical mechanical polishing technology forplanarization of deposited materials.

2. Description of the Related Art

The manufacture of an integrated circuit device requires the formationof various layers (both conductive and non-conductive) above a basesubstrate to form the necessary components and interconnects. During themanufacturing process, certain layers or portions of layers must beremoved to form the components and interconnects. Generally, the removalis achieved chemically (etching), or chemically and mechanically(chemical mechanical polishing).

FIG. 1 is a simplified cross-sectional schematic of a semiconductorwafer, consisting of a dielectric layer 102 on top of a silicon wafer104. Trenches are formed in the dielectric 102, using masking andetching techniques well known in the art. The dielectric 102 has a metallayer 101 deposited on top using techniques such as Physical VaporDeposition (PVD), Chemical Vapor Deposition (CVD) or electroplating.Copper is used in the metallic layer because of its inherent higherconductivity and improved resistance against electromigration, versusthe prior art aluminum. The deposited metal fills the previously createdtrenches 103. The metal above the plane of the dielectric must beremoved before subsequent steps in the device manufacturing process canbe performed.

One method of removing the excess metal is through CMP as illustrated inFIG. 1. A slurry 105 containing abrasive particles (not shown) isintroduced into a polishing device containing a polishing pad. Themechanical movement of the pad and abrasive particles relative to thewafer, combined with the chemical reaction of the slurry with the coppersurface 107, provides the means for removing the exposed, oxidizedsurface of the copper layer 107. The chemical nature of the slurry used,along with that of the particles, depends on the type of material to beremoved.

FIG. 2 illustrates the wafer of FIG. 1 after polishing. CMP of metalscan be used to define vertical and horizontal wiring 203 insemiconductor wafers, such as a silicon wafer. This process requireshigh selectivity in removal rate of metals 203 versus dielectricsurfaces 202, such as a silicon dioxide layer, to avoid both oxideerosion 206 on patterned structures and copper “dishing” 205, wheredishing is defined as selective localized removal of the copper versusthat of the surrounding oxide dielectric 202. This localized removaloccurs because of differing hardness between the oxide dielectric 202and the softer copper 203, and because the slurry is generally selectedto preferentially remove the copper over the dielectric. Oxide erosionis also a result of the aforementioned reasons; in this case, however,narrow oxide features are less resistant to the induced abrasive forcesthan the wider oxide features because of the “dishing” of copper lineson either side of the oxide.

There are a number of ways of improving selectivity toward metals.Process parameters that are varied to improve selectivity toward metalsversus dielectrics include reducing the polish pressure, optimizing therotational and orbital speed of the polishing device, selecting theproper slurry chemistry, polish pad material, and polish pad groovegeometry. However, all of these methods address the polishing side ofthe problem, rather than the material-choice issue for interlayerconnects.

An alternative to changing processing parameters is to change the natureof the material polished. A problem associated with CMP of copper layersin semiconductors is related to the inherent softness of copper. If thecopper could be hardened with little or no change in electrical orelectromigration properties, process selectivity adjustments may bereduced or eliminated, thus improving process robustness andsemiconductor device quality.

What is needed is an interlayer connect material for semiconductordevices that is harder than that used in the current art, allowing theuse of the present polishing materials and methods, but resulting inbetter control of oxide erosion and dishing. This material shouldmaintain close to the same conductivity and electromigration advantagesassociated with copper.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and notlimitation, in the Figures of the accompanying drawings in which:

FIG. 1 shows a prior art CMP polishing process.

FIG. 2 illustrates dishing and dielectric over-etch associated with theprior art CMP polishing process.

FIG. 3 a shows a side view of an embodiment of a metal hardening processusing solid solution hardening.

FIG. 3 b shows an embodiment of a metal hardening process usingprecipitation hardening.

FIG. 4 a illustrates the deposition of oxygen from an ambient source ona copper matrix with an aluminum solute.

FIG. 4 b illustrates the formation of an oxide dispersion-hardened metallayer by heating the metal film deposited in FIG. 4 a.

FIG. 5 illustrates the process used to form an oxide dispersion-hardenedsputtering target using a prior art powder metallurgy method.

DETAILED DESCRIPTION OF THE INVENTION

A method for improving the performance of the chemical-mechanicalpolishing (CMP) process used in polishing semiconductor interconnectlayers is described. In the following description, numerous specificdetails are set forth such as material types, dimensions, etc., in orderto provide a thorough understanding of the present invention. However,it will be obvious to one of skill in the art that the invention may bepracticed without these specific details. In other instances, well-knownelements and processing techniques have not been shown in particulardetail in order to avoid unnecessarily obscuring the present invention.

A method for reducing the topography from CMP of metal layers during thesemiconductor manufacturing process is described. Small amounts ofsolute are introduced into the conductive metal layer before polishing,resulting in a material with electrical conductivity andelectromigration properties that are very similar or superior to that ofthe pure metal, while having hardness that is more closely matched tothat of the surrounding oxide dielectric layers. This may allow forbetter control of the CMP process, with less copper dishing and oxideerosion as a result. A secondary benefit of this invention may be theelimination of superficial damage and embedded particles in theconductive layers caused by the abrasive particles in the slurries.

This discussion will mainly be limited to those needs associated withimproving CMP performance on copper layers deposited on silicon dioxidedielectric layers. It will be recognized, however, that such focus isfor descriptive purposes only and that the apparatus and methods of thepresent invention are applicable to other types of materials that may beused in constructing semiconductor devices.

A number of copper hardening methods are known in the art, but these aretypically used for bulk copper materials rather than the thin films usedin semiconductor processing. One method currently practiced for bulkcopper is solid solution hardening. In this method, a small amount(typically between 0.01–5.0%) of another metallic species (also referredto as a solute) is introduced into the copper matrix. The solute atomsmay take the place of the copper atoms in the normal copper matrix, oroccupy interstitial sites in the crystal lattice. The solute atoms may“pin” dislocations and vacancies within the matrix, thus preventingslippage between the various atomic planes of the metal, resulting inimproved mechanical properties (i.e. hardness and strength). Typicalsolutes used with copper solid solution strengthening are beryllium,silver, aluminum, zinc, zirconium, and chromium. Note that this list isnot exhaustive and the elements listed can be used singly (i.e. binarysystem) or in various combinations (i.e. ternary, quarternary, andhigher systems) depending on the material property that is to beenhanced.

FIGS. 3 a and 3 b show a cross-sectional view of a wafer to illustratean embodiment of the current invention. In this embodiment, a layer ofcopper 301 a is deposited over a layer of silicon dioxide material 302a, which is used in a semiconductor device as a dielectric. In oneembodiment, the thickness of the copper may be 5000 Angstroms. Thedielectric has had vias or contacts 303 a etched through it, to allowfor the interconnection of the various layers of the device. Rather thanusing pure copper, as would be done under the current art, a smallamount of a solute metal 305 a is introduced during the deposition. Theamount of solute used may be in the range of 0.01–5.0 atomic percent.Note, however, that the amount of solute used may differ substantiallyfrom this amount, and will depend on the matrix/solute system beingused. Therefore, the above amounts are for illustration only, and shouldnot necessarily be construed as limiting. The deposition methods usedmay include many used in the current art. Examples include CVD, PVD, orelectroplating, as discussed in the Background. The solute will beco-deposited along with the matrix metal. The solute 305 a in thisembodiment is beryllium; however, in another embodiment, either usingcopper or another matrix material, the solute species may be some otherspecies, such as magnesium or other appropriate element.

A second hardening method, precipitation hardening, is related to theabove solid solution method. In this method solute atoms are introducedduring deposition of the copper matrix, as was described above.Generally, precipitation strengthened metals involve higher solutepercentages compared to solid solution strengthening, 0.1–10% dependingon the matrix/solute phase. In this case, however, the material isheated to allow increased solution of the solute phase into the matrix.When the heated metal is cooled, the increased solute phase, which isunstable at ambient temperatures, precipitates out as a finely dispersedsolute-rich phase, which can sometimes be an intermetallic compound(e.g. CuBex). The precipitates impart higher yield strength to the metalby hindering dislocation motions in the metal lattice, resulting in astronger and harder material. Beryllium and aluminum are two examples ofpossible solute species.

Thus, in another embodiment of the present invention, the matrix metallayer with the included solute atoms (beryllium, in this embodiment) 301a is deposited as in the embodiment shown in FIG. 3 a. Once thedeposition is completed, however, the wafer is subjected to anotherheating step, as shown in FIG. 3 b. This “aging” heat treatment drivesfurther solute diffusion and consolidation to form “islands” 305 b ofberyllium-rich precipitates with a relatively large grain size. Thisapproach, while yielding a harder material than “pure” copper, leads toa softer material than the embodiment discussed previously. One possibleadvantage of this method is that it would allow for a range ofhardnesses to be achieved, thus allowing for more precise matching ofmetal hardness with that of the dielectric.

A third hardening technique often used in the art is oxide dispersionhardening. One method is to introduce a solute into the copper matrix,as discussed in FIG. 3 above. The solute is then oxidized, either by aninternal oxidant, such as an oxidized analog of the matrix material(i.e., a copper oxide for copper matrices), or by using an externalsource, such as an ambient oxygen supply. The solute is chosen so thatit will preferentially oxidize over the matrix material. A typicalsolute used is aluminum. Typical concentrations for solutes range fromabout 0.01% to 5.0% by weight solute metal.

FIG. 4 illustrates an embodiment of the invention using the oxidedispersion-strengthening technique. In this embodiment, a layer ofcopper containing a small percentage (for example, 0.01 to 5.0% byweight) of aluminum atoms is deposited on top of the dielectric 402 aand/or silicon wafer 404 a surface. The wafer containing the depositedfilm is then heated in an oxygen-containing atmosphere. The oxygenmigrates 407 b into the metal matrix 401 b. The oxygen 407 bpreferentially oxidizes the included aluminum atoms. This leads to theformation of dispersed aluminum oxide species 409 b, which act asdislocation barriers to slippage between the planes of the matrix. Thisresults in a stronger and, more importantly, harder material, with apossible electrical conductivity approaching 90% that of pure coppermetal. Note that while aluminum is used for the above illustration, anysolute with an appropriately high negative energy of oxide formation(versus that of the matrix metal) could be used. In general, the solutemetal should have a free energy of oxide formation at least 60kilocalories per gram of oxygen greater than that of the matrix metal.Examples of appropriate solute metals may include beryllium, magnesium,or thorium.

In a fourth embodiment, a sputtering target, used for PVD of metallayers in the semiconductor manufacturing process, can be manufacturedfrom powder metallurgy methods known in the art to form an oxidedispersion-hardened copper, similar to that discussed in FIG. 4 above.In this embodiment, however, the oxidant would be internal, using amixture of copper oxide (also referred to as the matrix metal oxide) andaluminum oxide (also referred to as the refractory metal oxide) for acopper target with an aluminum solute. FIG. 5 illustrates a typicalprior art processes flow for manufacturing oxide dispersion-strengthenedmetal powders. Copper oxide is heat reducible. By heating a mixture ofmetal alloy (a copper-aluminum alloy, for instance), matrix metal oxide,refractory metal oxide and solute 501 at approximately 950 C forapproximately 30 minutes 502, a metal matrix with internally oxidizedsolutes is formed. In this case, as in FIG. 4 above, the oxidized soluteformed is aluminum oxide. Some unreacted copper oxide may remain in thepowder following the oxidation step. A reduction step, at approximately800 C under hydrogen gas for approximately one hour 503, may be neededto reduce any unreacted copper oxide to the metallic state. Finally, theoxidized and reduced metal powder is annealed, in either an inertatmosphere or under vacuum, at approximately 850 C for about one hour504. This oxide dispersion-strengthened powder can then be formed into asputtering target 505 using methods known in the art. The target maythen be used to sputter a dispersion-hardened material directly onto thedielectric layer, eliminating the need for the elevated-temperatureoxidation step outlined in FIG. 4 above. This may result in a faster,less expensive process for the production of dispersion-hardened contactmaterials. As discussed above in FIG. 4, while aluminum was used as anexample of an appropriate solute for description of this embodiment,other solute metals with adequately negative free energies of oxideformation, such as beryllium, magnesium, or thorium may be appropriateto use in the embodiment described in FIG. 5.

Thus, what has been described is a method for improving chemicalmechanical polishing of copper layers in semiconductor devices.Application of the inventions described may significantly reduceunwanted metal loss incurred during CMP. In addition, copper CMP processconditions may be made less stringent which may allow potential materieland energy savings through reduced usage of consumables (polish pads,slurry, water, etc.)

In the foregoing detailed description, the apparatus of the presentinvention has been described with reference to specific exemplaryembodiments thereof. It will, however, be evident that variousmodifications and changes may be made thereto without departing from thebroader spirit and scope of the present invention. The presentspecification and figures are accordingly to be regarded as illustrativerather than restrictive.

1. A method for forming hardened interconnects comprising: depositing ametal film over a semiconductor wafer surface; introducing an additionalmetal species comprising beryllium to the metal film; heating thedeposited metal film with the introduced metal species; allowing themetal film to cool, so as to form precipitates of the introduced metalspecies; and after allowing said heated metal film to cool performingchemical-mechanical polishing wherein the additional metal precipitatesharden said deposited metal film to reduce the rate of said polishing.2. The method of claim 1, wherein the deposited metal film is copper. 3.The method of claim 1, further comprising introducing a solute selectedfrom the group consisting of silver, aluminum, zinc, zirconium,chromium, and magnesium before heating the deposited metal film with theintroduced metal species.